As is well known, expanded plastics are widely used as heat insulating materials, sound absorbing materials, floats, etc. Among known molding processes for obtaining expanded plastics is a foaming-in-mold method, in which preexpanded resin beads are put in a foaming mold, and heating steam is blown into the mold to heat and fuse the resin beads (see, for example, JP-B-3-56905, the term "JP-B" as used herein means an "examined published Japanese patent application").
Thermoplastic resin expanded plastics with a skin obtained by, for example, filling preexpanded resin beads in a hollow non-foam molded article and expanding the beads is known (see, for example, JP-B-42-10752). Appearance of expanded plastic with a skin has enlarged the uses of expanded plastics.
The expanded plastics with a skin has conventionally been produced by blowing a non-foaming parison in a blow mold, cooling and solidifying the blown parison to form a hollow article, removing the hollow article from the blow mold, filling the hollow article with preexpanded resin beads (hereinafter sometimes referred to as expanded beads), blowing heating steam from a steam injection pipe inserted among the filled beads to heat and further expand the expanded beads, and withdrawing the pipe before fusion of the expanded beads (see JP-B-62-19239 and JP-B-U-62-9073, the term "JP-B-U" as used herein means an "examined published Japanese utility model application").
According to the above-mentioned process, there are several problems. As the hollow article (skin) and the expanded plastic body are separately molded, separate molds are required and the process is complicated. Besides, the time for cooling to solidify the hollow article and that for heating and fusion of expanded beads and for cooling the expanded plastic body are independently required, resulting in poor productivity. In addition, since the expanded beads are molded in a once cooled hollow article, fusion of the expanded plastics to the hollow article tends to be insufficient.
In order to overcome these problems, it has been proposed to inject a reactive foaming liquid (e.g., urethane) into a parison being held in a closed blow mold, to blow the parison to the inner wall of the mold and then cool and solidify the blown parison with the mold closed while allowing the foaming liquid to react and solidify to obtain foam with a skin (see JP-B-58-10217 and JP-A-3-293113, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, according to the above-mentioned process, since blowing of the parison to the mold wall is effected by the foaming pressure of the foaming material, the blowing pressure is liable to be insufficient, failing to bring the parison into close contact with the inner wall, especially of the mold of complicated shape, and to uniformly inflate the parison.
JP-A-59-145125 proposes a process for producing expanded plastic with a skin comprising delivering a heated parison into a blow mold, closing the mold, blowing pressurized air from a pipe having been inserted in the parison for blow molding while feeding preexpanded beads with air into the parison, expanding the expanded beads by the heat of the parison, withdrawing the pipe, and cooling the article.
However, since the above process utilizes the heat of the parison for expanding of preexpanded beads, the heat is not sufficiently transmitted to the central portion of the expanded plastic body, resulting in the failure of uniform expanding and fusion, which leads to a reduction in expanded plastic quality.